In order to address questions related to human T-cell leukemia virus type I (HTLV-I) and bovine leukemia virus (BLV) infectivity such as receptor- binding, RNA packaging signals, roles of structural genes, etc., we have generated recombinant proviruses that contain reporter genes controlled by independent promoters. The recombinant proviruses may also be of value as vehicles for gene transfer into mammalian cells. We have constructed a recombinant BLV which contains the Neo-resistance marker and requires complementation with viral regulatory genes only. This recombinant virus can transfer Neo-resistance to a variety of cell lines by infection. A recombinant HTLV-I provirus was made containing the NEO-resistance marker and must be complemented with viral env and regulatory gene products to produce infectious progeny. In addition, we have constructed an HTLV-I helper virus to complement the recombinants. To overcome some of the problems associated with the use of the NEO-marker, we are constructing an HTLV-I recombinant provirus containing the bacterial lacZ gene. This will allow identification of cells expressing the lacZ gene in days rather than weeks and is compatible with cell sorting instrumentation.